feat(multi-atm): perfectly linear accrual

[adam-hotait]

Current situation

The MultiATM contract facilitates asset swaps (e.g., USTBL <> USDC) based on on-chain Oracle prices. To mitigate sandwich attacks during Oracle update blocks, the contract currently employs a spread-based logic:

• Buying USTBL (with USDC): Uses the maximum of the two most recent Oracle prices.
• Selling USTBL (for USDC): Uses the minimum of the two most recent Oracle prices.

This "min/max" logic prevents sandwich attackers from profiting from the price delta immediately before and after an Oracle update.

Multi-Hop Swaps. The contract supports swaps between any two assets provided an Oracle Path exists (e.g., eurSPKCC → EURC → USDC → USTBL). The price is calculated by traversing the vertices of the path, which is provided as a parameter by the user during the function call.

Objective of the feature: Linear Yield Accrual

We aim to introduce instant yield accrual for specific assets that exhibit linear Net Asset Value (NAV) growth, such as MMF shares (e.g., USTBL, EUTBL). For these Linear Accruing Pairs, we will replace the min/max logic with a linear extrapolation based on the current block timestamp. This allows for a single, fair price for both buys and sells that reflects real-time yield growth between Oracle updates. For such identified pairs, the price function need to be updated to:

$$Price_{t} = P_{latest} + (T_{current} - T_{latest}) \cdot R_{accrual}$$

Where:

• $P_{latest}$: Most recent Oracle price.
• $T_{current}$: Current block timestamp.
• $T_{latest}$: Timestamp of the most recent Oracle update.
• $R_{accrual}$: The rate of accrual derived from the two most recent Oracle data points.

[Copilot]

Pull request overview

Adds an optional “linear yield” pricing mode to MultiATM, where the swap price is derived via linear extrapolation from the last two oracle rounds (instead of min/max bounding), and extends the test suite to validate the new behavior.

Changes:

• Extend pair configuration and events to include a linearYield flag.
• Implement linear extrapolation pricing path in MultiATM using the two latest oracle rounds.
• Update and expand tests to cover linear-yield pricing behavior and updated pair details/event args.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 10 comments.

File	Description
contracts/token/MultiATM.sol	Adds linearYield to pair config, updates ABI (event + view), and introduces linear extrapolation pricing logic.
test/main.test.js	Updates existing expectations for the new flag and adds extensive regression/swap tests for linear-yield pricing.

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